1. Field of the Invention
The present invention is related to a universal input switching power supply, and more particularly to a universal input switching power supply having a parallel and serial type DC to DC converter to increase transforming efficiency.
2. Description of the Related Art
The universal input switching power supply may use a high line voltage range (185V to 265V) of an AC power source or a low line voltage range (90V to 130V) of an AC power source and provides a constant output voltage of a DC power source. Every country sets a normal high and low line voltages of the AC power, for example, in Taiwan, the normal high line voltage is 220 VAC and the normal low line voltage is 110 VAC.
With reference to FIG. 11, a conventional universal input switching power supply mainly has a full bridge rectifier (50), a power factor corrector (hereinafter PFC) circuit (51) and a DC to DC converting circuit (52). The full bridge rectifier (50) is coupled to the external AC power source (AC IN) and converts the AC power source (AC IN) to a first DC power source. Since the PFC circuit (51) is connected to the full bridge rectifier (50), a voltage of the first DC power source is boosted to a second DC power source (Vbulk) with higher voltage if the PFC circuit (51) uses a structure of a boost converter. On the contrary, the voltage of the first DC power is decreased when the PFC circuit (51) uses a structure of a buck converter. Using the boost converter as the PFC circuit (51) for an example, a PFC controller (511) of the PFC circuit (51) drives a power switch (S1) to turn on or off by outputting a PWM signal. The voltage of the second DC power source from the PFC circuit (51) will be close to 400V. The 400V of the voltage of the second DC power source is further supplied to the DC to DC converting circuit (52). A PWM controller (521) of the DC to DC converting circuit (52) outputs a PWM signal to a power switch (S2) to adjust current value of a primary side of a transformer (522). Pulse widths of the PWM signal from the DC to DC converting circuit (52) is modulated according to an output voltage (VO) of the universal input switching power supply. Therefore, the DC to DC converting circuit (52) stabilizes the output voltage (VO) of the universal input switching power supply at different voltages of the AC power source conditions.
The PFC circuit (51) of the above universal input switching power supply outputs 400V of the voltage of the second DC power source to the DC to DC converting circuit (52) when the universal input switching power supply is coupled to 220V AC power source (AC IN). Therefore, the DC to DC converting circuit (52) converts the 400V of the voltage of the second DC power source to a 12V or 5V of a voltage of a third DC power source. However, a transforming efficiency of the universal switching power is not good, especially coupling to 110V AC power source (AC IN). That is, the PFC circuit (51) has to converts the 220V or 110V of the voltage of the AC power source to 400V DC power source (Vbulk). In general, the PFC controller (51) using the structure of the boost converter has about 96% transforming efficiency at using 220V AC power source condition. However, the PFC controller (51) modulates a large pulse width of the PWM signal to the power switch (S1) to output 400V DC power source when the universal input switching power supply is coupled to the 110V AC power supply. Therefore, a conductive term of the power switch (S1) is increased, a conductive resistor of the power switch (S1) consumes more energy of the AC power source.
Accordingly, an transforming efficiency of the PFC circuit (51) coupled to the 110V AC power source is decreased to about 94% that is lower than that of the PFC circuit coupled to the 220V AC power source.
Another type of the universal input switching power supply, a PFC circuit uses a structure of a buck converter. The PFC circuit outputs 80V of the voltage of the second DC power source to a DC to DC converting circuit. A transformer of the DC to DC converting circuit is smaller than that of the above DC to DC converting circuit for boost converter. Therefore, the universal switching power efficiency is also not good, especially coupling to the high voltage of the AC power source. In general, the PFC circuit has to convert the 220V or 110V of the voltage of the AC power source to 80V of the DC power source through the full bridge rectifier. The PFC controller using the structure of the buck converter has 96% transforming efficiency at using 110V AC power source condition. On the contrary, an transforming efficiency of the PFC circuit coupled to the 220V AC power source is about 94% and is lower than that of the PFC circuit coupled to the 110V AC power source.
Based on foregoing description, each of the universal switching power supplies can not provide good transforming efficiency for different line voltages of the AC power sources.
To overcome the shortcomings, the present invention provides a universal input switching power supply to mitigate or obviate the aforementioned problems.